Glass is used as a substrate for magnetic disk storage media, which disks are installed in a drive device of personal computers and the like. This is because glass substrates are advantageous in that their impact strength is high and they have increased smoothness as compared with aluminum substrates. In recent years, to cope with demands for higher recording density, the distance between the magnetic head and the magnetic disk substrate has been progressively reduced. Accordingly, the glass substrate for magnetic disks necessarily has high precision flatness, a low surface roughness and is nearly free of flaws.
For polishing various glasses, materials such as cerium oxide, zirconium oxide, iron oxide and silicon dioxide have been conventionally used. At present, abrasive compositions mainly comprising cerium oxide (hereinafter referred to as "cerium oxide-type abrasive compositions") are predominantly used due to their high polishing efficiency.
With respect to the abrasive composition for polishing the glass substrate of a magnetic disk, the following techniques have been proposed in which additives are added to an abrasive mainly comprising cerium oxide, zirconium oxide or aluminum oxide to adjust the slurry properties and thereby improve the polishing efficiency and polishing precision.
(1) JP-A-3-146584 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") entitled "Abrasive for Polishing Glass" proposes to incorporate calcium aluminate, magnesium sulfate or magnesium chloride into an abrasive mainly comprising zirconium oxide.
(2) JP-A-3-146585 entitled "Abrasive for Polishing Glass" proposes to incorporate magnesium chloride into an abrasive mainly comprising cerium oxide.
(3) JP-A-9-109020 entitled "Composition for Polishing Magnetic Disk and Polishing Solution Using the Same" proposes to incorporate gibbsite and a dispersant into an abrasive mainly comprising aluminum oxide.
There is a great demand for glass substrates for magnetic disks having further improved surface precision. For stably achieving a low glide height as a property required for the glass substrate of a magnetic disk, conventional abrasive compositions cannot ensure the requisite low surface roughness and a clean glass surface that is free from attachments.
As described in the foregoing, in order to achieve higher magnetic disk recording density, it is essential to reduce the glide height of the magnetic head. Therefore, irregularities in the magnetic disk profile must be removed with higher precision. When a crystallized glass or a tempered glass substrate used as the glass substrate of a magnetic disk is polished using a conventional cerium oxide-type abrasive composition, a problem arises in that a low surface roughness necessary for reducing the glide height of the magnetic head to the required level cannot be obtained. Furthermore, because the cerium oxide-type abrasive compositions have high chemical reactivity with glass, attachments remain on the surface of the glass substrate of the magnetic disk. The attachments are difficult to thoroughly remove even by applying thereto mechanical energy such as ultrasonic cleaning or scrub cleaning, and the magnetic head contacts the residual attachments. Thus, the magnetic disk surface substantially fails to obtain the high precision needed for reducing glide height.